applying geomorphological principles and engineering...

Applying geomorphological principles andengineering science to develop a phased SedimentManagement Plan for Mount St Helens,WashingtonPaul Sclafani,1 Chris Nygaard1 and Colin Thorne2*1 Portland District, US Army Corps of Engineers, Portland, OR USA2 School of Geography, Nottingham University, Nottingham, UK

Received 29 April 2016; Revised 16 October 2017; Accepted 18 October 2017

*Correspondence to: Colin Thorne, School of Geography, Nottingham University, Nottingham, UK. E-mail: [email protected] is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,provided the original work is properly cited.

ABSTRACT: Thirty-seven years post-eruption, erosion of the debris avalanche at Mount St Helens continues to supply sediment tothe Toutle–Cowlitz River system in quantities that have the potential to lower the Level of Protection (LoP) against flooding unaccept-ably, making this one of the most protracted gravel-bed river disasters to date. The Portland District, US Army Corps of Engineers(USACE) recently revised its long-term plan for sediment management (originally published in 1985), in order to maintain the LoPabove the Congressionally-authorized level, while reducing impacts on fish currently listed under the Endangered Species Act, andminimizing the overall cost of managing sediment derived from erosion at Mount St Helens. In revising the plan, the USACE drewon evidence gained from sediment monitoring, modelling and uncertainty analysis, coupled with assessment of future LoP trends un-der a baseline scenario (continuation of the 1985 sediment management strategy) and feasible alternatives. They applied geomorpho-logical principles and used engineering science to develop a phased Sediment Management Plan that allows for uncertaintyconcerning future sediment yields by implementing sediment management actions only as, and when, necessary. The phased planmakes best use of the potential to enhance the sediment trap efficiency and storage capacity of the existing Sediment Retention Struc-ture (SRS) by incrementally raising its spillway and using novel hydraulic structures to build islands in the North Fork Toutle River(NFTR) and steepen the gradient of the sediment plain upstream of the structure. Dredging is held in reserve, to be performed onlywhen necessary to react to unexpectedly high sedimentation events or when the utility of other measures has been expended. Theengineering-geomorphic principles and many of the measures in the phased Sediment Management Plan are transferrable to othergravel-bed river disasters. The overriding message is that monitoring and adaptive management are crucial components of long-term sediment-disaster management, especially in volcanic landscapes where future sediment yields are characterized by uncertaintyand natural variability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

KEYWORDS: disaster management; gravel-bed river; Mount St Helens; North Fork Toutle River; sediment management; volcanic eruption

Introduction

The catastrophic eruption of Mount St Helens (MSH) on 18May 1980 altered the surrounding landscape both physicallyand ecologically (Swanson and Major, 2005), with thecatchment of the North Fork Toutle River (NFTR) being themost severely affected (Lipman and Mullineaux, 1981; Jandaet al., 1984). Impacts were greatest in the upper basin of theNFTR, but affected the entire Toutle–Cowlitz drainage systemand part of the Columbia River (Figure 1). In the yearsimmediately following the 1980 eruption, erosion of materialdeposited during the event generated sediment loads thatwere two to three orders of magnitude greater than pre-eruption levels. Thirty-seven years later, the average annualsediment load input to the Toutle–Cowlitz drainage system

is still about 10 times greater than it was prior to theeruption.

The principal, long-term risk posed to downstreamcommunities by the persistently elevated sediment load stemsfrom sedimentation in the lower Cowlitz River, which increasesthe probability of flooding in river-side communities includingCastle Rock, Lexington, Kelso and Longview (Figure 1).Recognizing this, soon after the eruption the United StatesCongress authorized the Portland District, US Army Corps ofEngineers (USACE) to manage sediment in the Toutle–Cowlitzsystem as necessary to ensure that the Level of Protection(LoP) provided by levees along the lower Cowlitz River ismaintained at or above the value prescribed in theCongressional authorization throughout a 50-year period thatbegan in 1985.

EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms (2017)© 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/esp.4277

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In responding to the eruption and elevated flood risks in theToutle–Cowlitz system during the years immediately followingthe event, the USACE undertook emergency sediment manage-ment actions and conducted a series of rapid scientific and en-gineering studies, culminating in an original SedimentManagement Plan (SMP) (USACE, 1983) and Decision Docu-ment (USACE 1985). Implementation of the original SMP wasbased on a series of feasibility studies and design memorandapublished during the 1980s (USACE, 1984, 1986a, 1986b,1987a, 1987b). The centrepiece of the original SMP was con-struction of the Sediment Retention Structure (SRS), which isan earth embankment across the NFTR approximately 22 kmdownstream of MSH (Figure 1). The SRS was completed in1989 and it trapped more than three quarters of the sedimentinput from the upper NFTR until sediment accumulating inthe sediment plain upstream of the structure reached the crestof the spillway, in March 1998.By 2009, it had become apparent from the results of on-

going monitoring of annual sediment yields from the upperbasin, sedimentation rates in the lower Cowlitz River anddeteriorating trends in the LoP, that continuing with the originalSMP was no longer feasible. Thus, further studies wereconducted to update the geoscience and engineeringknowledge bases for managing sediment in the Toutle–Cowlitzsystem, and then revise the original SMP. This paper reportshow these more recent studies led to development andadoption of a revised SMP that uses phased and adaptive

sediment management actions as needed to ensure that theLoP will exceed the Congressionally-authorized value up toand beyond 2035, despite persistence of annual sedimentyields from upper NFTR that are elevated compared to pre-eruption levels, highly sensitive to the occurrence of floods,and unpredictably variable.

The Eruption and Sediment ManagementResponses (1980–2009)

The eruption and initial emergency responses(1980–1984)

At 08:32 on 18 May 1980 a magnitude 5 earthquake triggereda massive landslide comprised of three failure blocks on thenorthern flank of MSH (Figure 2) (Voight et al., 1981; Glicken,1996). Seconds later, a lateral volcanic blast and ground-hugging pyroclastic flow followed the landslide, devastating a600 km2 area to the north of the mountain and blanketing itwith tephra (Hoblitt et al., 1981; Voight et al., 1981; Waittet al., 1981). A thick layer of volcanic ash was then depositedover a wide area and a series of lahars and floods redistributedsome of the debris from the eruption along the valleys of theToutle and Cowlitz Rivers, extending as far downstream asthe Columbia River (Figure 1). In the months and years

Figure 1. Location map showing Mount St Helens, the Toutle–Cowlitz drainage system extending to the Columbia River, the extent of the debris av-alanche, lakes dammed by the debris avalanche, locations of theN-1 dam, Sediment Retention Structure (SRS) and Fish Capture Facility (FCF), and com-munities along the lower Cowlitz River that are at risk of flooding. Note that most of the debris avalanche itself lies within the Mount St Helens VolcanicMonument, which eliminates options for erosion and sediment source control. [Colour figure can be viewed at wileyonlinelibrary.com]

P. SCLAFANI ET AL.

© 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Earth Surf. Process. Landforms, (2017)

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following the eruption, fluvial processes and more laharseroded sediment from the devastated area at enormous rates,depositing it in the Toutle, Cowlitz, and Columbia Rivers.During the eruption, the upstream 27.4 km of the NFTR were

buried to an average depth of about 40m by a layered depositreferred to as the debris avalanche. This wedge-shaped deposithas a volume of about 2.5 km3 and comprises (from top tobottom) of layers of: volcanic ash (fine-grained, highly erodible,generally up to 3m thick, but in places as much as 11m);pyroclastic flow material (mostly fine-grained but with up to20% gravel, low-density, highly erodible, generally up to11m thick but as much as 49m thick close to the mountainon the pumice plain); landslide deposits (ranging from

fine-grained sediment to massive boulders, much less erodiblethan either ash or pyroclastic flow material, up to 140m thickclose to the mountain) (Glicken, 1996).

The initial influx of sediment from the eruption reduced thedischarge capacities of the channels of the Toutle and CowlitzRivers (Schuster, 1983), putting the communities of Toutle,Castle Rock, Lexington, Kelso, and Longview (Figure 1) atsignificant risk of flooding, even during normal flows.Sedimentation also continued to pose a serious navigationhazard in the Columbia River. In response, a range ofemergency measures was immediately implemented by thePortland District, USACE (Willingham, 2005). These measuresincluded:

• construction of debris dams across the North and SouthForks of the Toutle River (the site of the N-1 dam on theNFTR is indicated in Figure 1) to reduce the volume ofsediment being delivered to the Toutle River;

• raising of levee crest elevations at vulnerable locations alongthe lower Cowlitz River between Castle Rock and Longview(Figure 1);

• dredging of the Columbia River around the mouth of thelower Cowlitz to eliminate the navigation hazard (Schuster,1983);

• dredging of the Toutle–Cowlitz drainage system thatremoved ~5.7 million m3 of sediment from these rivers,between December 1980 and May 1981 alone.

The N-1 dam had already filled with sediment when it wasdamaged by a lahar and then decommissioned, in 1982(Simon, 1999) and sedimentation persisted in the lower ToutleRiver, with an additional ~2.3 million m3 being dredged duringthe winter 1982–1983. When a further ~3.4 million m3 ofsediment had to be dredged from around the confluence ofthe Toutle and Cowlitz Rivers during the winter 1983–1984, itwas apparent that continued dredging alone would not providea long-term solution to managing sediment in theToutle–Cowlitz system.

The original sediment management plan (SMP)(1985)

When it was recognized that elevated sediment yields fromthe upper NFTR were likely to persist for decades, the PortlandDistrict began work on a long-term SMP, which was issued asthe ‘Decision Document’ in 1985 (USACE, 1985). Learningfrom the short life of the small N-1 sediment dam on theNFTR, the central component of the 1985 Plan was a muchlarger SRS located at a suitable site 22 km downstream of MSH(Figure 1). The SRS is an earth embankment (crest length =575m, height = 55m) with a roller-compacted, concrete up-stream face and a design sediment storage capacity of ~200million m3 (Figure 3). The intention was for the SRS to trapand retain sediment throughout the remainder of the 50-yearperiod for which the USACE was authorized to managesediment at MSH. Construction began in 1987 and the SRSwas completed in 1989.

The 1985 SMP also specified further improvements to leveesalong the lower Cowlitz River and made provision for thedredging to be continued during construction of the SRS andresumed when, at a future but unspecified date, sedimentaccumulating upstream of the SRS blocked the pipes releasingwater through the structure, diverting it instead over thespillway and causing the SRS’s trap efficiency to decrease asa result (USACE, 1985).

Figure 2. Sequence of events during the first minute of the eruptionon 18 May 1980. (A) pre-eruption condition, (B) massive, three-blocklandslide, (C) lateral blast and pyroclastic flow, (D) debris avalancheburying upper course of North Fork Toutle River (NFTR) to a maximumdepth of 140m. Note: Following the initial event a layer of ash was de-posited over the area around the volcano, and lahars and shallow-waterfloods redistributed debris avalanche and pyroclastic flow sedimentsdownstream through the Toutle–Cowlitz system as far as the ColumbiaRiver (see Figure 1) (courtesy of USGS Cascades Volcano Observatory).

GEOMORPHOLOGICAL PRINCIPLES FOR PHASED SEDIMENT MANAGEMENT


Sediment management under the original SMP(1985–2008)

During the 1990s, the SRS trapped 75 to 95% of the sedimenteroded from the debris avalanche (Figure 4), but accumulatingsediment progressively buried the structure’s outlet pipes andduring the 1996 flood water passed over the spillway for thefirst time. In March 1998 sediment accumulating upstream ofthe SRS reached the elevation of the spillway (Major et al.,2009) and the structure’s trap efficiency gradually decreased,falling from over 75% during the 1990s to about 30 to 40%during the early-2000s (Figure 4). This allowed more sedimentto pass downstream into the Toutle–Cowlitz system andrenewed sedimentation in the lower Cowlitz River wasdocumented, especially during and following a spike insediment transport in the NFTR that was related to thegeomorphological impacts of a flood in 2006 that triggered achannel avulsion in Loowit Creek – a headwater stream thatswitched from draining into Spirit Lake to become a tributaryto the NFTR (Figures 5 and 6; Simon and Klimetz, 2012).Following the 2006 flood, a marked loss of conveyance

capacity caused at least in part by the influx of avulsion-relatedsediment to the lower Cowlitz triggered actions necessary to

maintain the authorized LoP. These actions included reneweddredging and additional local levee improvements. Althoughthese measures were successful in preventing the LoP fromfalling below its Congressionally-authorized value, the low trapefficiency of the SRS observed between 2002 and 2008(Figure 4) and challenges posed by continued dredging of thelower Cowlitz River indicated that the LoP for some communi-ties along the lower Cowlitz might fall below the authorizedlevel as early as 2018 unless further sediment managementactions were undertaken.

Review of the original SMP and sediment expertworkshop (2009)

Although the 1985 SMP recognized that dredging in theCowlitz River would be required after the outlet pipes wereblocked and the SRS became a ‘run-of-river’ structure, by thetime dredging actually became necessary in the mid-2000s ithad become both more expensive and more difficult to permitthan originally envisaged. Furthermore, listing under theEndangered Species Act (ESA) of fish populations inhabiting

Figure 3. Sediment Retention Structure (SRS) and sediment plain in winter 2012–2013 (following implementation of the first spillway raise, insummer, 2012) (courtesy of USGS Cascades Volcano Observatory). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 4. Record of changes in the trap efficiency of the Sediment Retention Structure (SRS) in relation to key events including sediment storedupstream reaching the original spillway crest (March 1998), construction of the Grade Building Structure (GBS) pilot project (summer 2010) andimplementation of the pilot spillway raise (summer 2012). [Colour figure can be viewed at wileyonlinelibrary.com]

P. SCLAFANI ET AL.




the Toutle–Cowlitz system [including chinook salmon(Oncorhynchus tshawytscha), coho salmon (Oncorhynchuskisutch), chum salmon (Oncorhynchus keta), steelhead trout(Oncorhynchus mykiss), eulachon (Thaleichthys pacificus)and green sturgeon (Acipenser medirostris)] meant that oppor-tunities for dredging were limited to a few weeks each year.However, the original SMP anticipated the need for periodic

re-evaluation of its components in response to changes in localconditions and legislative contexts, and specific provision wasmade for periodic updating and revision (USACE, 1985). Actingon this provision, in 2009 the Portland District initiated areview of the original SMP and organized a Sediment ExpertWorkshop to consider options for future sedimentmanagement. At the workshop, experts considered 16 optionsfor sediment management having potential to help meet theauthorized LoP through to 2035 and beyond, while improvingconditions for fish and minimizing the overall cost of managingsediment (Table I). The outcomes of the 2009 workshopinformed subsequent screening of these 16 potential sedimentmanagement options and revision of the 1985 SMP.

Monitoring, Modelling and ForecastingSediment Yields (1982–2009)

Context

The 2009 workshop identified forecasting future trends inannual sediment yield as an issue to be addressed in revisingthe long-term plan for managing sediment in the Toutle–Cowlitz system. To address this, the Portland District com-piled and reviewed all available data on sediment yieldssince the 1980 eruption, and then undertook numericalmodelling to provide an engineering-forecast of futuresediment yields suitable for sediment management planningpurposes.

Initial estimates of the 50-year cumulative sedimentyield (1983–1985)

Immediately post-eruption, sediment yields from the debrisavalanche were truly monumental, and the USACE’s initialestimate of the 50-year cumulative sediment yield being inexcess of 750 million m3 reflected this (USACE, 1983). Duringthe next two years, a rapidly declining trend in annual sedimentyield emerged and in 1985 the initial estimate was reviseddown to ~420 million m3 (USACE, 1985). This estimate wasbased on the assumption that the rapid decline in annualsediment yield observed during the first few years followingthe eruption would continue, albeit at an exponentiallydecaying rate, as had been observed during geomorphic‘recovery’ or ‘relaxation’ in other fluvial systems followingdisturbance (Graf, 1977). The revised estimate of the forecastedyield made in 1985 was the basis for selecting construction of asingle, very large structure – the SRS – coupled with dredging ofthe lower Cowlitz as necessary, as the preferred option in theoriginal SMP (USACE, 1985).

Sediment monitoring and interpretation (1985–2009)

The original SMP was formulated on the expectation thatelevated sediment yields to the NFTR would decline throughtime and that sediment loads in the Toutle–Cowlitz systemwould be monitored and interpreted to make available thedata needed to characterize sediment transport rates andtrends (USACE, 1985). In this context, during the late-1990s,scientists at the US Geological Survey, Cascades VolcanoObservatory (CVO) analysed post-eruption suspendedsediment measurements in the Toutle River, together withUSACE’s records of sediment accumulation upstream of theSRS since its completion in 1989. CVO’s scientists then usedthe combined data to construct a synthetic time series ofannual sediment loads at the US Geological Survey (USGS)Kid Valley gauge (located just downstream from the SRS)between 1982 and 1998 (Major et al., 2000). Based on thesedata and records from other fluvial systems disturbed byvolcanic activity Major et al. (2000, p. 822) noted that:

Sediment yields in the aftermath of explosive volcaniceruptions typically decline nonlinearly as physical and vege-tative controls diminish sediment supply. However, spatialand temporal perturbations resulting from hydrologicfunctions are likely to punctuate, or even temporarilyreverse, long-term trends, which complicates projection oftime to equilibrium.

Figure 6. Annual sediment yields to the Sediment Retention Structure(SRS). The 1996 and 2006 floods can be seen to interrupt the longer-term decline in sediment yields, indicating that the fluvial systemremains sensitive to disturbance by flood events. The 1996 event wasthe flood of record. The 2006 flood had a return period of only~20 years but generated a spike in sediment loads due to an avulsionin Loowit Creek – a headwater stream in the North Fork Toutle River(NFTR) basin. Note: This plot uses ‘water years’ that start on 1 Octoberand end on 30 September. [Colour figure can be viewed atwileyonlinelibrary.com]

Figure 5. Sedimentation in the lower Cowlitz River related to the2006 flood. Such deposition has the potential to rapidly andsignificantly reduce the Level of Protection (LoP) provided by the lowerCowlitz floodway and levee system. [Colour figure can be viewed atwileyonlinelibrary.com]





TableI.

Three-levelscreen

ingofoptio

nforsedim

entman

agem

ent(U

SACE,

2010)

Measures

Statusafterfirstscreen

ing

Statusafterseco

ndscreen

ing

Statusafterthirdscreen

ing(Lim

ited

Re-evaluationRep

ort,LR

R)

1.Deb

risavalan

chestab

ilizatio

nNotv

iable

–treatm

entw

ould

soonbescouredan

davalan

che

ismostly

with

intheMountSt

Helen

s(M

SH)National

Volcan

icMonu

men

t

2.ElkRock

sedim

entdam

Notviab

le–would

store

sign

ificantsedim

entbutmuc

hmore

costly

than

raisingtheSedim

entReten

tionStructure

(SRS)

3.GradeBuildingStructures(G

BSs)

Viable–somesedimen

tstorage;low

costan

den

vironm

entimpa

ctViable

Viable

–pilo

tproject

provesGBSfeasibility

4.Su

mpin

sedim

entplain

Notviab

le–dueto

highco

stan

dlim

itedsedim

enttrap

ping

andstorage

potentia

lRe-introducedin

response

inthirdscreen

ingin

response

topublic

commen

t

Notv

iable–further

analysisshowsthisto

be

very

costly

andineffective

5.Raise

SRSdam

andspillway

Viable

–relativ

elyhighco

stbutsign

ificantsedim

enttrap

ping

andstorage

potentia

lViable

Viable

–butv

eryhighco

stsuggestsmeasure

#6isabetteroptio

n

6.Increm

entalraisingofSR

Sspillway

Notviab

le–moderatesedim

entim

pact

forlow

costbut

conc

ernsexistregardingcapacity

ofSRSto

conv

eyProbab

leMaxim

um

Flood(PMF)

(whichisadeb

risflo

w)

Viable–becau

seupdated

PMFmodelsshow

that

deb

risflo

wsdonotreachtheSR

S

7.Ban

kstab

ilizatio

n(in

cludinglower

Toutle

-1dredge

spoilsite,LT-1)

Notv

iable

–ban

kerosionnota

sign

ificantsourceofsed

imen

t.However,loc

alban

ktreatm

enta

tLT-1dredge

spoildisposal

site

beco

nsidered

Re-introducedin

thirdscreen

ing

inresponse

topublic

commen

t

Notviab

le–inputofsedim

entfrom

erosion

atLT-1

smallco

mpared

tothesedim

ent

budgetforTo

utle

–Cowlitzsystem

8.Su

mpat

lower

Toutle

dredge

spoilsite

(LT-1)

Viable

–further

assessmen

tofsedim

enttrap

effic

iency

required

totestfeasibility

inphase3

Viable

Notviab

le–co

stly

andlim

itedcapa

city

totrap

sandsizedsedim

entthat

dep

ositsin

lower

CowlitzRiver

9.Expan

dflo

odplain

ofTo

utle

River

Notviab

le–lim

itedopportunities

toexpan

dtheflo

odp

lain

andlim

itedsedim

entim

pact

10.Modify

operationofMossyrock

Dam

Viable

–reco

mmen

dad

ditional

evaluationofpotentia

lto

pass/flu

shsedim

entthrough

lower

CowlitzRiver

Viable

–relia

bility

ofap

proach

unprovenbutstill

worth

considering

Notviab

le–lim

itedan

dunrelia

ble

for

passing/flu

shingsedim

entthrough

lower

Cowlitz

11.Leveeim

provemen

tsNotviab

le–would

increase

floodrisk

innon-leveedreache

s.Minorim

provemen

tsstill

appropriate

thou

gh

12.CowlitzRiver

dredging

Viable

–effectivebutco

stly

plusen

vironmen

tally

sensitiv

eViable

–effectivebutwith

environmen

talan

ddisposal

issues

Viable

–butneedto

limitfreq

uen

cyan

dextent

13.Expa

ndflo

odp

lain

oflower

Cowlitz

Viable

–further

considerationwarranted

Notviab

le–dueto

extentof

floodplain

developmen

tan

dinfrastructure,plusco

stof

removingco

ntam

inated

soils

14.H

orseshoeben

dsumporcu

toffin

lower

CowlitzRiver

Notviab

le–lim

itedben

efitformajorco

stpluspotentia

lfor

findingco

ntaminated

soils

15.Recon

nectold

chan

nel

atmouth

of

CowlitzRiver

Notviab

le–lim

itedben

efitformajorco

stpluspotentia

lfor

findingco

ntaminated

soils

16.Dikes

toflu

shsedim

entat

mou

thof

CowlitzRiver

Viable

–further

considerationwarranted

Viable

Not

viable–somebene

fitbu

tstro

ngly

rejected

dueto

impa

ctson

listedfish

andwild

life

Note:Thefirst,seco

ndan

dthirdscreen

ings

correspond

toinitial,feasibility-level

andpracticality

-level

evaluations.

P. SCLAFANI ET AL.


In interpreting the implications for sediment management,they concluded that:

… yields from basins that experience dominantly channeldisturbances will likely remain elevated for as much asseveral decades. Thus, measures designed to mitigatesediment transport in the aftermath of severe explosiveeruptions must remain functional for decades, p. 822.

We now know that elevated sediment yields in the NFTRhave indeed persisted, primarily due to post-disturbancechannel evolution characterized by geomorphically complexresponse (Schumm, 1977). Such response involves cycles ofincision, aggradation and widening, with channel changesbecoming increasingly dependent on the occurrence of floodsdue to the combined effects of slope adjustments, bedarmouring/fining, bank instability, and lateral channel erosion(Simon and Thorne, 1996; Major et al., 2000; Zheng et al.,2013, Zheng et al., 2017). In this context, Major et al. (2000)correctly predicted that:

If bank instability persists, high sediment yield persists.

WEST Consultants (2002) performed further analysis of thecombined USGS and USACE sediment records. They coupledthe river hydrograph with a one-dimensional sedimenttransport model (HEC-RAS; Brunner, 2001) to characterize thedecreasing trend in annual sediment yields observed between1982 and 1999. They concluded that this trend had the formof an exponential decay curve with the equation:

y ¼ 21:2 x0:6

where y is the annual sediment input to the sediment plainupstream of the SRS (in t/yr) and x is the time elapsed sincethe eruption (in years). Based on extrapolating their best fit lineto 2035, WEST Consultants (2002) recommended furtherdownward revision of the 50-year, cumulative sediment yieldto ~310 to ~320 million m3.Notwithstanding this, Major (2004) used his calculation that

two decades of erosion following the eruption had removedonly ~12% of the sediment deposited in the debris avalancheto support a further cautionary statement to the effect that:

Persistent extraordinary suspended sediment yields fromseverely disturbed channels indicate that mobile supplies ofsediment remain accessible, and those supplies will not beexhausted for many more years or possibly decades.

Biedenharn group engineering study (2008–2009)

Biedenharn Group (2010) provided support for the hypothesisthat decay in sediment yields from the debris avalanche mightbe slower than initially thought. Their study calculated erosionby differencing digital elevation models (DEMs) representingparts of the upper NFTR basin on various dates between 1984and 2007. These DEMs were derived from conventionalsurveys conducted in the years immediately following theeruption and a subsequent series of aerial LiDAR (lightdetection and ranging) surveys. It should be noted that mostof these DEMs provide only partial coverage of the debrisavalanche. To support long-term sediment managementplanning, the Biedenharn Group (2010) adopted a conservativeassumption that the decreasing trend in annual sediment yieldsobserved immediately following the eruption actually ended

during the late-1980s. Their characterization of the trend incumulative erosion between 1990 and 2007 consequentlyhad the form of a straight line with the equation:

y = 4.5 x.where y is the cumulative erosion (in U.S. tons) andx is the calendar year since the eruption. Based on this linearrelationship, the Biedenharn Group (2010) provided aconservative engineering estimate of the 50-year cumulativesediment yield of ~560 million m3.

Post-2009 Expert Workshop Studies and PilotProjects (2009–2012)

USDA-ARS study (2009–2011)

Following review of the available data and contrasting forecastsfor future sediment yields at the Sediment Expert Workshop in2009, a new study was commissioned by the Portland Districtto establish the potential for further decay in future sedimentyields from the debris avalanche. This study, undertaken bystaff at the US Department of Agriculture, Agricultural ResearchService (USDA-ARS) National Sedimentation Laboratory atOxford, Mississippi, coupled re-surveys of 30 of the USGS-monumented cross-sections in the upper NFTR drainagesystem with advanced modelling of bank and terrace slopeinstability using the Bank Stability and Toe Erosion Model(BSTEM) (Simon et al., 2011).

In the USDA-ARS study, Simon and Klimetz (2012) used dataderived from long-term monitoring performed by the CVO (fora summary, see Mosbrucker et al., 2015) together withadditional data they collected themselves to concur with theconclusion reached by CVO scientists that, post-2000, bankerosion had become the dominant source of sedimentdelivered to the SRS. They also highlighted the growingsignificance of floods, citing as evidence channelchanges/avulsions in headwater tributaries like Loowit Creek,and spikes in annual sediment yields observed during andfollowing the floods of February 1996 and, particularly,November 2006 (Figure 6).

Notwithstanding flood-related disturbances to the long-term,declining trend, Simon and Klimetz (2012) forecast that, unlessthe fluvial system were to be perturbed by further volcanicactivity, long-term sediment yields to the SRS should beexpected to decay logarithmically according to the function:

y ¼ 69:3 ln xð Þ þ 53:7

where y is the annual sediment yield (in tons) and x is the timeelapsed since the eruption (in years), indicating a 50-yearcumulative sediment yield of ~330 million m3

Grade building structure pilot project (2010–monitoring is ongoing)

The 2009 Sediment Expert Workshop considered 16 measuresthat could theoretically be taken to manage sediment in theToutle–Cowlitz system (Table I). Following post-workshopscreening in 2010, the Portland District took the decision to testthe efficacy of Option #3 in Table I: construction of gradebuilding structures to retain additional sediment on thesediment plain upstream of the SRS. A cutoff berm, cross-valleystep-weir and baffle structure, and 14 Engineered Log Jams (ELJs)were installed upstream of the SRS in a pilot project intended toestablish their potential to increase the gradient of the sedimentplain and induce the formation of vegetated islands (Figure 7).



Monitoring and performance appraisal of the pilot projectcontinues and will do so until the structures are, in due course,buried by sediment accumulating upstream of the SRS.

Pilot spillway crest raise (2012)

When constructed (between 1987 and 1989), the SRS was fittedwith a spillway of sufficient capacity to protect the earth em-bankment against over-topping during the Probable MaximumFlood (PMF). At that time, the PMF was not envisaged to be aconventional water flood, but rather a debris flow caused bybreaching of the debris-avalanche dam impounding Castle Lake(Figure 1). The spillway was therefore sized to safely pass such adebris flow (USACE, 1990). In 2010–2011, PMF models wereupdated to reflect changes to the geometry of the NFTR valleycaused by sediment accumulation upstream of the SRS (USACE,2011b; Denlinger, 2012). Four scenarios were modelled: twosimulating water floods from different breaching modes of theCastle Lake debris-avalanche dam, the third simulating a debrisflow caused by breaching of debris-avalanche dam at CastleLake, and the fourth simulating a debris flow originating fromthe crater of MSH. The results indicate that the probable maxi-mum debris flow emanating from either the crater or Castle Lakewould not reach the SRS spillway due to the width of the valleyof the upper NFTR and the breadth of the sediment plain up-stream of the SRS (USACE, 2011b; Denlinger, 2012). A waterflood resulting from a ‘worst case scenario’ breaching of CastleLake would produce a peak flow of 40 000m3/s but this wouldbe attenuated to just 6000m3/s by the time it reached the SRS.This discharge is much lower than the original design flood forthe spillway. The high degree of attenuation is again attributedto the great width of the upper valley and breadth of thesediment plain. These outcomes led to downward revision ofthe PMF (USACE, 2012) which, in turn, allowed raising the crestelevation of the SRS spillway without compromising its safety, asa management option.The efficacy of raising the spillway in reducing sedimenta-

tion in the lower Cowlitz was tested in the summer of 2012(USACE, 2013). The spillway was raised by ~2.3m, using adesign that did not steepen it or adversely affect downstream

migration by juvenile, anadromous fish or the potential forfuture re-establishment of volitional passage upstream byreturning adults (Figure 8). Subsequent sediment monitoringshows raising the spillway restored the trap efficiency of theSRS to over 60% (Figure 4), and bathymetric surveys in thelower Cowlitz River indicate that sedimentation that beganwhen the SRS began operating as a ‘run-of-river’ structure inMarch 1998 has been reversed (Figure 9).

USACE Forecast of Future Annual SedimentYields (2012)

Modelling approach and outcome

In light of continuing uncertainty in sediment yield predictions(USACE, 2009), the Portland District decided to undertakefurther modelling to develop a ‘Future Expected DepositionScenario’ (FEDS) (USACE, 2011a). To ensure that sedimentyield forecasts were conservative, it was assumed that therewould be no significant decrease in the average annual rate oferosion of the debris avalanche during the period up to 2035.

In forecasting future sediment yields to the SRS, the PortlandDistrict compiled as complete a time-series of measureddischarges and estimated sediment yields as possible for theperiod between March 1998 (when the SRS became a‘run-of-river’ structure) and 2007, based on their own recordsplus those of the CVO and the USDA-ARS. They then used aMonte-Carlo approach to synthesize multiple time-series ofpossible annual sediment yields, starting in 2008 and endingin 2035.

Thousands of possible annual sediment yield and down-stream deposition series were generated, creating a spectrumof possible sediment futures that was statistically stable. Asingle, simulated time-series representing the median fluvialfuture in terms of both future average annual discharges andsediment yields was then designated as the ‘FEDS’. Theselected time-series has a distribution of floods and sedimentyield events similar to those observed between 1990 and 2007(Figure 10). This similarity indicates that events in the time

Figure 7. Grade building structures installed in the 2010 Pilot Project. Inset orthophoto shows (a) cutoff berm, (b) cross-valley step-weir and bafflestructure, and (c) island building structures. Main picture shows Engineered Log Jam island building structures in area outlined in red in the orthophotothree years after their installation. [Colour figure can be viewed at wileyonlinelibrary.com]

P. SCLAFANI ET AL.



series are representative of floods actually experienced in theToutle–Cowlitz system. Based on the median fluvial future,and solely for the purpose of long-term sediment managementplanning, the Portland District USACE developed an engineer-ing estimate of ~130 million m3 for the cumulative sedimentyield from erosion of the debris avalanche during the periodbetween 2008 and 2035.Although the Portland District’s ‘FEDS’ makes no allowance

for climate change, a subsequent study (Meadows, 2014)considers a range of possible hydroclimatic future scenariosfor the upper NFTR catchment, derived from a databasecompiled by the Columbia Basin Climate Change ScenariosProject (CBCCSP) (Hamlet et al., 2010, 2013; Tohver et al.2014). Under this range of possible hydroclimatic futures, thecellular-automata landscape evolution model CAESAR-LisFlood (Coulthard et al., 2013) was used to forecast long-termsediment yields up to 2100 and beyond. Long-term futureaverage annual sediment yields forecast using CAESAR-

LisFlood range between ~2.5 and ~4.5 million m3/yr (Meadows2014; Figure 10), resulting in a forecast that the cumulativesediment yield in 2035 is likely to be between 200 and 300million m3. This range reflects uncertainty regarding the effectsof climate change and natural variability, while also allowingfor model uncertainty.

Recognizing uncertainty and natural variability infuture sediment yields

Considerable uncertainty remains regarding future sedimentyields to the SRS (Figure 10). It must be accepted that thisuncertainty is, to a degree, irreducible. Also, inter-annualvariability in sediment yields is apparent in the historical recordand this must be expected to continue. However, it alsoremains uncertain how the sensitivity of sediment yields to

Figure 8. Pilot raise of the Sediment Retention Structure (SRS) spillway under construction during summer 2012. [Colour figure can be viewed atwileyonlinelibrary.com]

Figure 9. Rates of cumulative sedimentation in the lower Cowlitz River between 2003 and 2013. The impacts of various measures can be discerned.The 2010 Grade Building Structure pilot project slows deposition (compare deposition between August 2009 and July 2010 to that between July 2010and August 2012), and the 2012 spillway raise reverses sediment deposition in the lower Cowlitz River (i.e. cumulative erosion observed betweenAugust 2012 and September 2014). [Colour figure can be viewed at wileyonlinelibrary.com]





the occurrence of floods will evolve in future, and the timingand frequency of future events is simply unknowable. There-fore, it is impossible to predict when and how often spikes insediment delivery to the lower Cowlitz River (like thoseobserved in 1996 and 2006) will generate abrupt declines inthe LoP that may trigger the need for emergency dredging.Thus, the best practical response to managing uncertainty

and natural variability in sedimentation events in the lowerCowlitz is to adopt an adaptive approach to long-termsediment management. This is not only desirable environmen-tally, it is also beneficial economically. Applying an overlyconservative assumption (for example that floods will be fre-quent and sediment yields commensurately high), results inover-investment in building new sediment management infra-structure that probably is not needed. Conversely, assumingthat future sediment yields will decline significantly maynecessitate later implementation of emergency actions thatare both costly and environmentally damaging. In contrast,an adaptable approach not only delays capital expenditureon new works until they are actually needed, but also avoidsthat expenditure altogether if future sediment yields are lowerthan forecast and late-stage components in the SMP proveunnecessary.

Revising the Long-term SMP

Options appraisal

Having identified options for sediment management and fore-cast future annual sediment yields, the Portland District turnedits attention to revising the 1985 SMP (USACE, 2014). Thisprocess was addressed in three phases. In a first screening ofoptions identified at the 2009 Expert Workshop (Table I), teamscomposed of USACE staff and regional stakeholders (includingstaff from other Federal and State agencies, officials fromCowlitz County and members of the communities affected)assessed their viability with respect to:

• significantly reducing flood risk to communities along thelower Cowlitz River;

• cost-effectiveness;• adverse environmental/ecological impacts (including those

on listed species);• reliability of design;• adaptability to changing conditions;

• protection of cultural resources;• public acceptability.

Initial screening ruled out nine options, mainly due to short-comings relating to reliability of design; cost-effectiveness; andcapacity to significantly reduce flood risks (Table I). Full detailsof the first screening can be found in USACE (2010).

A second screening performed on the seven remainingpotential options involved:

• production of conceptual designs;• limited hydrologic, hydraulic, and sediment transport

modelling;• development of refined cost estimates.

The second screening eliminated option #13 (Table I).Appendix A of USACE (2010) fully explains the analysesperformed for each option considered in the second screening.

The third screening evaluated the practical feasibility of theremaining six options, plus two that had been screened out inphase 2 but which were reconsidered in response to publiccomments and representations (options #4 and #7 in Table I),and #6, which was reintroduced because revised PMF modelsshowed that debris flows would no longer reach the SRS.

The four options that remained after the third screening were:

#3: install further grade building structures (GBSs) on thesediment plain upstream of the SRS.

#5: raise the elevation of the SRS (including its spillway).#6: raise the elevation of the SRS spillway, but not the

embankment itself.#12: dredge the lower Cowlitz River.

These four options were grouped into three (numbered)action alternatives for revising the long-term SMP:

1 Rely solely on dredging the lower Cowlitz River (option#12).

2 Raise the elevation of the SRS dam and spillway (options #5and #6).

3 Phased implementation of GBSs, spillway raises, anddredging as and when necessary (options #3, #6 and #12).

These action alternatives were then evaluated in the LimitedRe-evaluation Report (LRR) (USACE, 2014) with respect to:

Figure 10. Observed sediment yields from the debris avalanche up to 2014 and predicted in the ‘Future Expected Deposition Scenario’ (FEDS)modelled by the Portland District (USACE, 2011a), together with long-term trends in sediment yields forecast in various studies. [Colour figure canbe viewed at wileyonlinelibrary.com]

P. SCLAFANI ET AL.



• effectiveness in maintaining the authorized LoP in the lowerCowlitz;

• whole life cost (i.e. capital and maintenance costs averagedover the 18-year period between implementation of therevised plan in 2017 and 2035);

• environmental/ecological impacts (including those on listedspecies);

• reliability of design and operation;• adaptability and potential for phased

construction/implementation.

Appraisal of the three action alternatives (Table II)established action alternative 3 – phased implementation ofspillway crest raise, GBSs, and dredging, as the best actionalternative.

Preferred action alternative

In 2014, the Portland District indicated in its draft long-termsediment management planning document that phasedimplementation of options #3, #6 and #12 had beenprovisionally selected as the preferred action alternative formaintaining the authorized LoP in the lower Cowlitz River,and the relevant Environmental Impact Statement was issuedthe following year (USACE, 2015).In the order in which they would be implemented, these

options are:

#6: Raise the spillway of the SRS by up to 7.3m in two furtherincrements (neither involving an elevation rise greaterthan 4m) as and when sediment stored upstream of theSRS reaches the current elevation of the spillway(which was raised by 2.3m in the 2012 pilot project).

#3: Following each of the two remaining spillway raises, ifand when sediment again reaches the spillway crest,construct further GBSs to retain additional sedimentupstream on the sediment plain.

#12: Dredge the lower Cowlitz River on an ‘as needed’ basis.Dredging may be needed, for example, in response torapid accretion during and following an event thatgenerates a short-term spike in sedimentation or afterthe SRS again becomes a ‘run-of-river’ structure but nofurther spillway raises are possible and construction ofadditional GBSs cannot be justified.

These options are to be implemented only as and whenneeded, which conserves adaptive capacity during the life ofthe project, delays the costs and impacts of necessary actions,and avoids the costs and impacts of actions that turn out to beunnecessary. By selecting phased implementation as the pre-ferred action alternative, USACE accepted that monitoring ofhydrologic and sediment conditions at the SRS and in the lowerCowlitz is essential for reliable decision-making.The maximum cumulative increase in spillway crest eleva-

tion is limited to 7.3m owing to the need to safely pass thePMF. Also, engineering constructability dictates that futureraises are best performed in two stages. Notwithstanding theseconstraints, the timings of the remaining incremental spillwayraises are not prescribed a priori, and optimum partitioning ofthe 7.3m raise between the two increments need not beidentified until such time as conditions trigger the next raise.Adaptive capacity allows flexibility in decision-making, andallows construction to be synchronized with the funding cyclefor sediment management actions.Each time sediment reaches the spillway crest and

sedimentation in the lower Cowlitz threatens to lower the LoP

below its authorized value, a sediment management action(e.g. construction of further GBSs on the sediment plain) maybe triggered. However, the phased plan recognizes that bythe time this happens sediment yields to the SRS may havediminished sufficiently that further management actions areunnecessary.

However, if elevated sediment yields have not diminishedsufficiently, and the LoP deteriorates as a result of reducedSRS trap efficiency, this will trigger construction of further GBSson the sediment plain. The effectiveness of a cross-valley step-weir and baffle structure (intended to store sediment) and 14ELJs (intended to induce island formation) in building up thegrade of the sediment plain was tested at the prototype scalein the 2010 GBS Pilot Project (Figure 7). The island-buildingstructures currently continue to perform as designed, promptingthe genesis and growth of islands that divide the flow andpromote proto-floodplains that are colonized by pioneeringvegetation, roughen the sediment plain, reduce the sedimenttransport capacity of the NFTR and, hence, build up thegradient of the sediment plain. Conversely, scour immediatelydownstream of the 2010 cross-valley structure has promptedresearch into an alternative design that would train the NFTRinto a ‘serpentine’ (i.e. tortuously meandering) course (Ettemaet al., 2016).

If serpentine river training structures are implemented, theNFTR will flow around the structures at its natural gradient, sothat the structures would not inhibit fish passage. Also, narrowgaps between the cross-valley structures and the valley sideswould protect existing ‘wall-base’ channels at the edges ofthe sediment plain. Wall-base channels are important tosalmon recovery in the Pacific Northwest because they havebeen shown to support both up and downstream passage whilealso providing excellent rearing habitat for juvenile fish(Peterson and Reid, 1984; Cederholm and Scarlett, 1991).

The ‘serpentine’ approach has recently been tested at ‘proofof concept’ level in a physical scale-model at Colorado StateUniversity (Figure 11) and the results are encouraging (Ettemaet al., 2016). If the serpentine approach to building grade inthe sediment plain is adopted, the Portland District will furtherevaluate, refine and optimize design of the necessary trainingstructures prior to implementation.

Based on the conservative, engineering forecasts of futureaverage annual sediment yields assembled by the USACE inthe ‘FEDS’ (Figure 10), two further spillway raises, coupled withconstruction of further GBSs when the SRS again operates as a‘run-of-river’ structure, are expected to maintain the LoP forcommunities along the lower Cowlitz at values higher thanthose Congressionally-authorized, at least up to 2035 andprobably beyond that date.

Nevertheless, it is still possible that sedimentation couldsignificantly reduce the LoP in the lower Cowlitz prior to2035. Acute sedimentation may occur during and following aflood that generates an unusually high input of sediment tothe Toutle–Cowlitz system, as experienced in 1996 and 2006(Figures 5 and 6). Renewed capital dredging in the lowerCowlitz River is therefore retained as an option to deal withsuch events, which are forecast to be relatively infrequent.Furthermore, should future annual sediment yields turn out tobe significantly higher than forecast, it is conceivable that thevolume of sediment by passing the SRS and settling in the lowerCowlitz may cause the LoP for some of the communities toapproach authorized values before 2035. In this unlikelyscenario, the option remains to renew maintenance dredginglate in the project. The extent of reaches requiring dredgingwould be limited to problematic shoals and bars, and thevolumes of sediment removed would be much smaller thanwould have been the case for the ‘Dredging Only’ alternative



TableII.

Evaluationofactio

nalternatives

intheLimite

dRe-evaluationRep

ort(U

SACE,

2014)

Criteria

Actionalternatives

1CowlitzRiver

dredgingonly

2Sedim

entReten

tionStructure

(SRS)

dam

andspillway

raise

3Phased

implemen

tatio

nof(spillway

raises

+Grade

Build

ingStructure

(GBS)

+dredging

asneeded

)

Effectiveat

maintainingLevelof

Protection(LoP)

Meetsneed

Meetsneed

Meetsneed

Economics–averagean

nual

cost

(over18years2017–

2035)

Highe

stco

stalternative.

Highupfront

costsbutlow

out-year

costs.

Least-co

stalternative.

Implemen

tatio

nco

stsdistributed

overtim

e,dredgingco

stsincu

rred

only

inresponding

toacutesedim

entdep

ositio

nan

dmay

notactually

be

requ

ired

.En

vironmen

talco

nsiderations

Rep

eated,freq

uentim

pactsto

hab

itatsan

dspecies

inriveran

dalso

inpotentia

lspoildisposalareas.

Adverseim

pactsonupstream

tributaries/hab

itats.

Future

upstream

fishpassage

attheSR

Sunlik

elyto

remainan

optio

n.

Upstream

tributary

hab

itatim

pactsan

dinterm

itten

tim

pactsin

lower

Cowlitzduringdredging

.Fu

ture

volitional

upstream

fishpassage

atSR

Sremainsan

optio

n.

Reliability

Reliable

ifdredgingcanbeperform

edwith

inco

nstraintsonen

vironm

entalim

pact,disposal,

andbudget.

Reliable

provided

that

monito

ringisco

ntinued

.Reliable

provided

that

monito

ringan

dphased

implemen

tatio

nareco

ntin

ued

.

Adap

tability

Highad

aptiv

ecapa

city

provided

dredgingof

lower

Cowlitzcanbeexpan

ded

–ifrequ

ired

tomeetbytheLo

P.

Noad

aptiv

ecapa

city:requ

ires

fullfund

ingan

dco

nstructionupfront

(whichmay

be

unnecessarily

conservative).

Optim

umad

aptability:co

mpo

nen

tsofplanim

plemen

ted

overtim

ean

donly

asan

dwhen

needed

.

P. SCLAFANI ET AL.


(i.e. action alternative 1, in Table II). In both acute and chronicscenarios for problematic sedimentation, dredging would beperformed only ‘as needed’ to maintain the LoP above theauthorized value.

Delivering the Revised SMP in Practice

Overview

Delivering the phased plan in ways that are economically,socially and environmentally acceptable, and which use thePlan’s adaptive capacity to best advantage, require a soundunderstanding of sediment dynamics within the Toutle–Cowlitzsystem and, particularly, the complex relationships betweenchanges in the quantity of sediment passing the SRS, morpho-logical responses in the Toutle–Cowlitz system downstream,and trends in the LoP. To foster this improved understanding,the results of on-going monitoring of sediment yields from thedebris avalanche, channel changes in the Toutle–Cowlitzdrainage network and trends in LoP are synthesized to informdecision-making.The phased plan responds to and relies on accurate

knowledge and assessment of trends in the LoP and identifica-tion of the appropriate sediment management action needed toprevent a breach of the authorized LoP. Given the lead timerequired for engineering implementation of structuralmeasures, a further requirement is that monitoring of trends inthe LoP triggers responses early enough to ensure that there istime for any action implemented to be effective in preventingthe LoP from falling below its authorized value.In applying the phased plan in practice, it must be borne in

mind that the Portland District’s management planning horizonis limited to a 50-year period that began in 1985. Thus, theUSACE has no authority to plan or perform sediment manage-ment actions that may be necessary to maintain the currentLoP beyond this planning horizon. But the District understandsand takes seriously cautions from sediment experts thatelevated sediment yields from the debris avalanche may persistfor many decades (e.g. Major et al., 2000; Meadows, 2014;Zheng et al., 2017). The phased SMP therefore includes

provision for further re-assessment in 2025 that will draw onthe results of on-going monitoring to elucidate the processesresponsible for trends in annual sediment yields, identifylong-term trends in those yields, and re-evaluate the hazardposed by sedimentation rates in the lower Cowlitz River in lightof the remaining sediment storage potential of the SRS.Monitoring, re-assessment and adaptation will ensure that thephased plan continues to meet the needs of communities alongthe lower Cowlitz River throughout the period of Congressionalauthorization, and into the foreseeable future.

Monitoring of the LoP

The primary goal of the phased plan is to ensure that communi-ties at risk of flooding receive at least the LoP authorized byCongress. The foundation for achieving this goal is carefulmonitoring of the lower Cowlitz River floodway and leveesystem to support a Management Decision Support Framework(MDSF) that draws on the answers to the following questions:

1 Is the LoP currently being met?2 Is there a declining trend in the LoP?3 Is the current assessment being made within five-years of the

end of the authorized period for sediment management, in2035?

Depending on whether the answers to these questions arepositive or negative, the MDSF considers the need forimplementation of a sediment management action via one ofthree links (Figure 12).

Identifying the need for a sediment managementaction

The decision to implement a sediment management action istriggered if the authorized LoP is not being met and there isno reasonable expectation that it will recover without a newsediment management action, or if there is reason to believe

Figure 11. Physical model built at Colorado State University to perform ‘proof of concept’ experiments using a series of cross-valley structures topromote aggradation and build up the gradient of the sediment plain by training the braided channel of the North Fork Toutle River (NFTR) into a‘serpentine’ course (Ettema et al., 2016). [Colour figure can be viewed at wileyonlinelibrary.com]




that the LoP will fall below the authorized value prior to 2035unless action is taken to prevent this (Figure 13).In situations where the LoP is declining, or the authorized

LoP is not being met, the state of the sediment managementsystem is assessed to predict whether the declining trend inthe LoP is likely to reverse or the sub-authorized LoP is likelyto recover to an acceptable value in the near future. Forexample, if a negative trend in LoP is detected following amajor sediment transporting event, that negative trend proba-bly reflects a flood-related spike in the sediment load input tothe lower Cowlitz River. In that case, it is reasonable to expectthe downward trend to cease or reverse once the sedimentwave passes through to the Columbia River and, therefore, no

new sediment management action is triggered. Conversely, ifthere is reason to expect that the LoP will not stabilize orrecover, and the authorized period is more than five-years fromcompletion, implementation of a new sediment managementmeasure is triggered (Figure 13).

If annual monitoring detects a precipitous fall in the LoP, or ifthe LoP is forecast to fall below the authorized value within twoor three years, a pre-emptive sediment management action istriggered. It is triggered because uncertainties concerningfuture floods, sediment loads, and budget/construction leadtimes require prudent action even though the LoP may stillexceed the authorized value. Justification for the investmentrequired to support such precautionary sediment management

Figure 12. Management Decision Support Framework (MDSF) Step 1. This step considers whether the authorized Level of Protection (LoP) is beingmet, whether there is a declining trend in the LoP and whether the period for which the Portland District USACE is authorized to manage sediment inthe North Fork Toutle River (NFTR) is within five-years of completion. Outcomes A (Red), B (Green) and C (Purple) lead to the matching, colour-codedquestions in Step 2 (Figure 13). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 13. Management Decision Support Framework (MDSF) Step 2. This step indicates to the river engineers and scientists responsible for man-aging sediment in the Toutle–Cowlitz system whether a sediment management action should be triggered or deferred. [Colour figure can be viewed atwileyonlinelibrary.com]

P. SCLAFANI ET AL.




actions rests not only on close monitoring of the LoP andsedimentation in the lower Cowlitz River, but also on regularsurveys of the sediment plain to evaluate the trap efficiency ofthe SRS.During the final five years of the authorized period, the need

for pre-emptive sediment management actions necessary toprovide sediment storage at the SRS beyond 2035 will beevaluated. If post-2035 sedimentation appears likely to beproblematic, then an action may be triggered between 2030and 2035 even though it is not actually needed to maintainthe LoP during the authorized period.

Selection of the appropriate sediment managementaction

Once the MDSF has triggered an action (Figure 13), it isnecessary to select and implement the sediment managementoption appropriate to the situation. This decision is made inthe third step of the MDSF, which offers three options: raisingthe SRS spillway, constructing further GBSs on the sedimentplain, or dredging critical reaches in the lower Cowlitz River(Figure 14).In deciding which sediment management option to imple-

ment, it is essential to match the action to the current situationin the Toutle–Cowlitz sediment transfer system. For example,if a wave of sediment (generated by a flood event or resultingfrom sediment dynamics in the NFTR) is migrating through thelower Toutle River, then raising the spillway or building newGBSs on the sediment plain cannot prevent a decline in theLoP. In this instance, if action is needed, the only viable op-tion is to dredge reaches of the lower Cowlitz floodwaywhere the LoP is at, or is trending towards, its authorizedvalue. Conversely, if cumulative deposition in the lowerCowlitz over a period of years is driving a declining trend inLoP, consideration is first given to an incremental raise ofthe SRS spillway, if such a raise is possible (Figure 8). If it isnot, consideration is given to construction of ELJs and river

training structures (Figures 7 and 11) to further increase thesediment storage capacity of the SRS.

A decision to dredge in response to chronic, cumulativesedimentation in the lower Cowlitz will be considered onlywhen the potential for trapping and storing additionalsediment upstream of the SRS has been fully exploited. Basedon current forecasts of future sediment yields from the debrisavalanche, this situation is unlikely to occur prior to 2035 –that is, within the period authorized for sediment manage-ment actions.

Discussion and Conclusions

Rivers draining catchments on the flanks of live volcanoes suchas Mount Pinatubo and Mount Merapi are responsible for someof the most deadly and enduring gravel-bed river disasters(Gran et al., 2011; De Bélizal et al., 2013). Managingvolcano-related sediment problems consequently presentspressing challenges to river scientists and engineers chargedwith protecting life and property in communities located involcanic landscapes and at risk of flooding, erosion orsedimentation.

These challenges are compounded by large uncertaintiesinherent to predicting long-term trends in future sedimentyields from landscapes and rivers disturbed by volcaniceruptions, which feature elevated sediment loads, heightenedmorphological-sensitivity to flood events and exaggeratedvariability in annual sediment yields from basins containingmassive amounts of highly erodible, volcanically-derivedsediment (Pierson and Major, 2014).

The reality is that plans and actions taken to managelong-term, sediment-related flood risks following an eruptionmust cope with irreducible uncertainties (i.e. uncertaintiesthat cannot be made significantly smaller using better science)and amplified natural variability that cannot be predicted, insediment futures that are, essentially, unknowable. Thiscontrasts markedly to other sediment-management contextsand authorizations, where planning and implementation take

Figure 14. Management Decision Support Framework (MDSF) Step 3. This set guides decision-makers with respect to the appropriate sedimentmanagement action on the basis of the wider context for the need for action and the options available. [Colour figure can be viewed atwileyonlinelibrary.com]




place in response to circumstances that are well-described,with hydrologic and sediment parameters that are stationary,at least in the near to medium-term.The original SMP for the Toutle–Cowlitz River system

draining MSH, Washington (USACE, 1985) has been revisedto formulate the current ‘Phased Sediment Management Plan’(USACE, 2014, 2015). The Phased Plan is designed to accom-modate uncertainty and natural variability by optimizing itsadaptive capacity and metering-out deployment of thatcapacity over the period for which implementation of thePlan is authorized. The Phased Plan is capable of maintainingthe authorized LoP afforded to communities along the lowerCowlitz River during the period for which the PortlandDistrict, USCAE is authorized to manage sediment in theToutle–Cowlitz system, no matter how the future unfolds. Italso minimizes costs by only implementing sedimentmanagement actions only as and when they are actuallynecessary, while applying a precautionary principle thatensures that the decision to trigger an action allows timenot only for the measure to be designed and constructed,but also for it to begin working effectively. The Phased Planis also designed to protect downstream fish passage and keepopen the possibility of restoring volitional upstream passagein the future.The adaptive, phased approach relies on monitoring and

annual appraisal of:

• flood and other sediment-related risks;• the basin-scale sediment transfer system;• the condition and performance of existing flood and

sediment infrastructure.

This is essential to inform river engineers and scientistsresponsible for deciding where and when to implementsediment management actions, as well as guiding selection ofthe option appropriate to address the particular sedimentproblem in the context of sediment dynamics in the widerfluvial system.To this end, the ‘Phased Sediment Management Plan’ uses a

three-part MDSF that prioritizes making best use of existingsediment-management infrastructure (especially the SRS), takesadvantage of working with natural processes by building up thegradient of the valley floor (sediment plain) upstream of the SRSusing GBSs, and reserves dredging as the management optionof last resort, to be used only in emergencies (i.e. in responseto acute sedimentation events driven by extreme floods) or afterthe potential of other options (SRS, GBS) has been fullyexploited.While the MDSF provides a structured and transparent basis

for triggering management actions and selecting appropriatemeasures, it is not a substitute for sound professional judgment,which is indispensable to delivering disaster management thatis prudent and safe. The reality is that decision-makers remainaccountable for the decisions they make. That said, the PhasedPlan employs geomorphic principles and engineering scienceto inform risk-based decision-making that is logical, cost-effective and explicable to stakeholders: it is difficult tooverstate the importance of these attributes in an age of intensepublic accountability.The underpinning geomorphological principles, engineering

science and many of the features of the Phased SMP aretransferrable to other gravel-bed river disasters. The over-ridingmessage is that monitoring and adaptive management shouldbe recognized as crucial to sustainable, long-term disastermanagement in gravel-bed rivers where future sediment yieldsare elevated, highly variable and characterized by greatuncertainty.

Acknowledgements—The authors thank Dr Vern Manville and,especially, Dr Jon Major for their detailed, insightful and constructivecomments on the first and second drafts of this paper, which benefittedboth its content and clarity. In part, this work was supported by theEngineering and Physical Sciences Research Council, UK [grant numberEP/P004180/1].

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https://doi.org/10.1130/2009.fld015(06)

https://doi.org/10.1130/2009.fld015(06)

http://eprints.nottingham.ac.uk/27800/1/Thesis_FINAL_TM.pdf

http://eprints.nottingham.ac.uk/27800/1/Thesis_FINAL_TM.pdf

https://doi.org/10.1016/j.geomorph.2013.11.018

https://doi.org/10.1016/j.geomorph.2013.11.018

applying geomorphological principles and engineering...

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